The present invention relates to a laser device for use as a light source in a protection-type exposure system.
Heretofore, light sources for emitting light beams to expose patterns for semiconductor integrated circuits have generally been high-pressure mercury lamps. A spectral line such as a g-line (436 nm) or an i-line (365 nm) of the radiation range of such a high-pressure mercury lamp has been used in the fabrication of LSI (large-scale integration) circuits. Fabricating SLSI (super-large-scale integration) circuits with much smaller patterns requires light sources which emit radiations of shorter wavelengths. One light source which can meet such a requirement is a laser beam source such as an excimer laser, for example. The excimer laser employs, as a laser medium, a mixture of a rare gas such as krypton or xenon and a halogen gas such as of fluorine or chlorine. The excimer laser can produce oscillation lines at a certain wavelength between 353 nm and 193 nm and with output powers which are large enough for pattern exposure.
The gain bandwidth of excimer lasers is broad, e.g., of about 1 nm. When an excimer laser is oscillated in combination with an optical resonator, its oscillation lines have a bandwidth (full width at half maximum) of about 0.5 nm. If a laser beam having such a relatively broad bandwidth is employed as an exposure light source, then the exposure optics used with the laser beam need to comprise a focusing optical system with corrected chromatic aberration, as is the case with an exposure light source in the form of a lamp. However, in an ultraviolet range whose wavelength is 350 nm or shorter, the chromatic aberration of the focusing optical system cannot fully be corrected since the available choice of optical materials for the lenses of the focusing optical system is limited.
If the bandwidth of oscillation lines of an excimer laser used in an exposure system can be reduced to about 0.005 nm for monochromatic laser emission, then it becomes possible to employ a focusing optical system with its chromatic aberration uncorrected. Therefore, the optical system of the exposure system is simplified, and the exposure system itself is reduced in size and cost.
In order to render a laser beam having a broad bandwidth monochromatic, the laser beam may be passed through a wavelength selection filter having a narrow passband. However, the wavelength selection filter greatly attenuates the output power of the laser, thus making it impossible to employ the laser as an exposure light source. One general solution has been to place a wavelength selection element within a resonator for making the laser beam monochromatic without attenuating the laser beam output power. FIG. 1 of the accompanying drawings shows such a conventional excimer laser having a narrow bandwidth. As shown in FIG. 1, a discharge tube 101 is placed in an optical resonator which comprises a totally reflecting mirror 102 and a partially reflecting mirror 104. The discharge tube 101 is filled with a medium gas that is composed of a mixture of a rare gas and a halogen gas, and that is excited by an electric discharge for laser oscillation. A wavelength selection element 105 which comprises a Fabry-Perot etalon is disposed in the optical resonator. In the excimer laser of the illustrated arrangement, only a light beam having a certain wavelength which is selected by the Fabry-Perot etalon 105 is amplified for laser oscillation. Therefore, the excimer laser can produce an output laser beam 106 having a highly narrow bandwidth and a high output power.
With the conventional laser device which has a narrow bandwidth, however, since a high-energy light beam present in the optical resonator passes through the wavelength selection element, the wavelength selection element tends to be distorted or deteriorated, resulting in a change in the selected wavelength or a drop in the laser output power. As a consequence, integrated circuits which are produced when the prior laser device is used as an exposure light source may become defective.